1. Field of the Invention
The invention pertains to the field of fluid separation using forward osmosis. Forward osmosis methods and apparatus using a supported osmotic agent to establish or enhance an osmotic forward bias are disclosed. A supported osmotic agent may be assisted by osmotic agents not attached to a support and/or a pressure differential between an influent and effluent chamber and/or a temperature gradient and/or other means to increase the osmotic pressure in an effluent chamber.
2. Description of Related Art
The use of semi-permeable membranes as a separation barrier between two solutions not in osmotic equilibrium is well known, and was first described in the French scientific literature in the mid 1700's. Such membranes permit passage of a solution's solvent but not its solute. A starting solution to be treated/filtered consisting of a certain solute molality is considered the influent. An ending solution after treatment/filtration consisting of a certain solute molality is considered the effluent. An osmotic differential between the two solutions exists when the molality of the effluent is different from that of the influent. To create a forward osmotic bias for the influent solvent, the effluent solute molality or osmotic potential must be greater than the influent solute molality or osmotic potential.
Molality refers to the number of solute molecules per liter of solution. In general, the greater the number of solute molecules in a solution, the greater is its osmotic pressure as compared to a solution lacking that solute. This solute differential creates an osmotic imbalance and natural forces of osmotic pressure drive solvent across a semi-permeable membrane separating the influent from the effluent until an osmotic equilibrium is reached between the two solutions. Take for example an influent that is fresh water and an effluent that is sea water; the solute is sea salt and the solvent is fresh water. Because the effluent has a higher molality of solute or osmotic pressure, fresh water will pass through the membrane to the effluent until an osmotic balance is reached.
Reverse osmosis, RO, accomplishes this objective by forcibly attempting to pass a solute-containing solution across a semi-permeable membrane whereby the membrane “filters” out the solute (e.g., sea salt, etc.) and passes the solvent to create solvent-only effluent (e.g., fresh water). However, reverse osmosis has a host of deficiencies including high energy requirements, membrane integrity during use and storage, and complexity.
In order for forward osmosis, FO, to work, the effluent, or draw solution, must have a solute molality or osmotic potential greater than the solute molality or osmotic potential of the influent, or feed solution. Problematically, however, the effluent is often fresh water (a common solvent) of exceptionally low solute molality. One solution used in prior efforts has been to benignly increase the effluent solute molality through the introduction of beneficial solutes such as carbohydrates and/or electrolytes. Thus, while accomplishing an objective of forward osmosis, e.g., the creation of potable water from non-potable water without the deficiencies of reverse osmosis, the effluent is not substantially pure, fresh water; it still contains the solute adjuncts or osmotic by-products.
The concept of forward or direct osmosis as a practical commercial process has been recognized since at least the 1930's. See, for example, U.S. Pat. No. 2,116,920. This patent discloses the use of a concentrated sugar and CaCl2 aqueous solution to “pull” water out of fruit juices. The general process has been in continuous commercial use to manufacture fruit juice concentrates since at least that time. The concept of a removable “driving solute” in forward osmosis driven separation is articulated by Charles Moody in his 1977 dissertation. He outlines the use of dissolved SO2 as an osmotic agent that would increase an effluent's molality above that of sea water, thereby creating a forward osmotic bias that would cause fresh water migration through a semi-permeable membrane from a sea water influent. The SO2 would then be removed from the effluent by increasing the effluent temperature to drive it out as a gas.
In U.S. Pat. No. 3,617,547, an approach similar to that of U.S. Pat. No. 6,391,205 is disclosed. In both cases, an osmotic agent composed of salts, whose solubility is very temperature dependent, is used to increase an effluent's molality above that of sea water, thereby creating a forward osmotic bias that would cause fresh water migration through a semi-permeable membrane from a sea water influent. The osmotic agent is removed by lowering the solution temperature to precipitate the solute out of solution. The precipitate is removed, re-dissolved in water aided by heating and then recycled. These processes are encumbered by the energy inefficient need to chill all of the permeate and recycle streams, as well as the need to reheat the recycle.
Keith Lampi et al. in U.S. Pat. No. 6,849,184 describe a novel approach of obtaining fresh water from impure or sea water by a combination of forward osmosis and reverse osmosis. Salt and sea water are introduced into a chamber with two semi permeable membranes and then sealed; the introduced solution, therefore, has a molality greater in comparison to that of ordinary sea water. Ordinary sea water is then exposed to a first one of the two semi permeable membranes, which causes the sea water solvent, i.e., fresh water, to cross the first membrane. As the fresh water passes through the first membrane and into the sealed chamber, the internal pressure of the sealed chamber increases.
Cascade Designs of Seattle discloses in published application, WO 2006/047577, May 4, 2006 a FO apparatus employing a protein/nanomagnetic complex of particles such as dried, powdered Magnetoferritin supplied by Nanomagnetics, Bristol, UK. The invention is directed to forward osmosis methods and apparatus employing at least one controllable osmotic agent. Basic apparatus embodying the invention comprise at least one semi-permeable hydrophilic or hydrophobic membrane as a separation barrier between a first fluid solution (influent), comprising a first solvent, and a second fluid solution (effluent) comprising a second solvent. To create a forward osmotic bias from the influent to the effluent, apparatus embodying the invention comprise at least one controllable osmotic agent added to the effluent to create an osmotic imbalance that favors migration of the first fluid solution solvent to the second fluid solution. The resulting osmotic imbalance permits the natural forces of osmotic pressure to drive the first solvent of the influent across the at least one semi-permeable membrane into the effluent until an osmotic equilibrium is reached between the two fluid solutions or the supply of influent ceases. Basic apparatus according to the invention may further comprise means for isolating, removing or neutralizing the at least one controllable osmotic agent from the effluent. A component of the methods and apparatus disclosed herein is a controllable osmotic agent. As used in the application, the term “controllable osmotic agent” is defined as a substance that alters the osmotic potential between a first fluid solution exposed to one side of a solvent semi-permeable membrane, and a second fluid solution exposed to the other side of the membrane, where the influence of the substance on the osmotic potential across the membrane can be manipulated. Thus, a controllable osmotic agent according to the invention is one that a) dissolves, or is suspendable in the second fluid solution such that it is able to establish or enhance an osmotic driving force across the membrane relative to the first fluid solution exposed to the other side of the membrane; and b) possesses at least one chemical or physical property, or combination of the two, that allows for its removal, neutralization or separation from the second fluid solution by means that do not appreciably affect the solvent of the second fluid solution. A controllable osmotic agent present in embodiments of the invention is one that is responsive to magnetic forces and/or electric fields, allowing it to be magnetically and/or electrically influenced, and thus separated from the second fluid through standard magnetic separation techniques that otherwise have no appreciable effect on the second fluid solution solvent. Other examples include, but are not limited to, osmotic agents that are removed/reduced through filtration, chemical precipitation, chelation, oxidation/reduction reactions, distillation, evaporation, pressure adjustments/manipulations, temperature adjustments/manipulations, electro-chemical means, capacitive deionization and other means known to those skilled in the art.
In recent bench-scale studies by McCutcheon and co-workers at Yale University, it was demonstrated that when using a suitable FO membrane (e.g. the FO CTA membrane) and a strong draw solution (highly soluble ammonia and carbon dioxide gases), seawater can be efficiently desalinated with FO. The draw solution was formed by mixing together ammonium carbonate and ammonium hydroxide in specific proportions. The salt species formed include ammonium bicarbonate, ammonium carbonate, and ammonium carbamate. Analysis of the process has shown that an osmotic pressure driving force (On) as high as 238 bar for a feed water with a salt concentration of 0.05 M NaCl, and as high as 127 bar for a feed water with a salt concentration of 2 M NaCl, can be achieved with the ammonia/carbon dioxide draw solution. This is a rather high driving force considering that 2 M NaCl is equivalent to brine from seawater desalination at approximately 70% water recovery.
In the novel ammonia-carbon dioxide FO process water is extracted from seawater and dilutes the ammonia-carbon dioxide draw solution; FIG. 1 schematically shows a desalination process based on this concept; note the “Draw solute recovery” unit. Upon moderate heating (near 60° C.), the draw solution decomposes to ammonia and carbon dioxide. Separation of the fresh product water from the diluted draw solution can be achieved by several separation methods (e.g., column distillation or membrane distillation (MD)). The degasified solution left behind is pure product water and the distillate is recovered draw solution available for reuse in the FO desalination process.
Bench-scale FO data demonstrates that the ammonia-carbon dioxide FO process is a viable desalination process. Salt rejections greater than 95% and fluxes as high as 25 L/m2 h were achieved with the FO CTA membrane with a calculated driving force of more than 200 bar. Although this is a relatively high flux, much greater flux is actually expected for such a high driving force. Further analysis of the results has indicated that the performance ratio (defined as experimental water flux divided by theoretical water flux) of the FO CTA membrane used was at most 20%, and on average between 5% and 10%. FIGS. 2a and 2b show current energy requirements for various desalination technologies as reported by McGinnis, et al.
All of the previous, yet limited, work on FO as an alternative desalination process has exposed the two major limitations of FO—lack of high-performance membranes and the necessity for an easily separable draw solution. Moreover, when considering seawater desalination, and especially when high water recovery is desired, FO can be utilized only if the draw solution can induce a high osmotic pressure.
In view of the foregoing, a forward osmosis process and related apparatus that produces an effluent having a solute concentration less than the concentration used during the osmotic process is desirable; preferably solute concentration of a permeate is quite small to non-detectable.